1. Field of the invention
The present invention relates to an arrangement for introducing a liquid medium, e.g. urea, into exhaust gases from a combustion engine.
2. Related art
To meet prevailing exhaust cleaning requirements, today's motor vehicles are usually provided with a catalyst in the exhaust line to effect catalytic conversion of environmentally hazardous constituents of the exhaust gases to environmentally less hazardous substances. A method which has been employed for achieving effective catalytic conversion is based on injecting a reducing agent into the exhaust gases upstream of the catalyst. A reductive substance which forms part of, or is formed by, the reducing agent is carried by the exhaust gases into the catalyst and is adsorbed on active seats in the catalyst, resulting in accumulation of the reductive substance in the catalyst. The accumulated reductive substance may then react with and thereby convert an exhaust substance to a substance with less environmental impact.
Such a reduction catalyst may for example be of the SCR (selective catalytic reduction) type. This type of catalyst is hereinafter called an SCR catalyst. An SCR catalyst reduces NOx in the exhaust gases.
In the case of an SCR catalyst, a reducing agent in the form of urea solution is usually injected into the exhaust gases upstream of the catalyst. The injection of urea into the exhaust gases results in the formation of ammonia which then serves as the reductive substance which assists the catalytic conversion in the SCR catalyst. The ammonia accumulates in the catalyst by being adsorbed on active seats in the catalyst, and NOx present in the exhaust gases is converted to nitrogen gas and water when it is brought into contact in the catalyst with accumulated ammonia on the active seats in the catalyst.
When urea is used as the reducing agent, it is injected into the exhaust line in the form of a liquid urea solution via an injector. The injector comprises a nozzle via which the urea solution is injected under pressure into the exhaust line in the form of a finely divided spray. In many operating conditions of a diesel engine, the exhaust gases will be at a high enough temperature to be able to vaporise the urea solution so that ammonia is formed.
It is difficult, however, to avoid part of the urea solution supplied coming into contact with and becoming attached to the internal wall surface of the exhaust line in an unvaporised state. The exhaust line, which is often in contact with and cooled by surrounding air, will be at a lower temperature than the exhaust gases within the exhaust line. When a combustion engine is run in a uniform way for a period of time, i.e. during steady-state operating conditions, no appreciable variations in the exhaust flow occur and the urea solution injected into the exhaust gases will therefore reach substantially the same region of the exhaust line throughout said period of time. The relatively cool urea solution may cause local lowering of the temperature in that region of the exhaust line, which may lead to the formation in that region of a film of urea solution which is then entrained by the exhaust flow. When this film has moved a certain distance in the exhaust line, the water in the urea solution will boil away under the influence of the hot exhaust gases. Solid urea will remain and will be slowly vaporised by the heat in the exhaust line. If the supply of solid urea is greater than the amount vaporised, solid urea will accumulate in the exhaust line. If the resulting layer of urea becomes thick enough, the urea and its decomposition products will react with one another to form urea-based primitive polymers known as urea lumps. Such urea lumps may over time block an exhaust line.
It is therefore desirable that the injected urea solution be widely spread out in the exhaust gases so that it is prevented from concentrating in substantially the same region of the exhaust line. A good spread of the urea solution in the exhaust gases also facilitates its vaporisation.
For reasons of space it is not usually possible to have a rectilinear exhaust line between a combustion engine and an exhaust outlet of a motor vehicle. Before the exhaust gases reach the mixing duct in which injected urea solution is mixed with them and becomes vaporised, they therefore usually pass through one or more pipe bends. Their passage through a pipe bend results in an oblique distribution of the exhaust flow as seen in the plane perpendicular to the direction of flow. Bringing an injected spray of urea solution into contact with such an obliquely distributed exhaust flow hinders the spreading of the urea solution in the exhaust gases and at the same time increases the risk that the urea solution might concentrate on substantially the same wall region in the mixing duct.